Full-SAP 18-hour off-peak tariff (code 5) must resolve to EIGHTEEN_HOUR 🟥

The full-SAP energy_tariff code space includes 5=off-peak-18-hour, but
_sap_17_1_meter_type only maps 1-4, dropping code 5 to "" → STANDARD.
Real SAP-19.1.0/19.2.0 certs lodge tariff 5, so the 18-hour off-peak
split is silently lost.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

datatypes/epc/domain/tests/test_from_sap_schema.py

@@ -181,6 +181,7 @@ class TestFullSapElectricityTariffTranslation:
             (2, "SEVEN_HOUR"),       # off-peak 7-hour → Economy 7
             (3, "SEVEN_HOUR"),       # off-peak 10-hour → dual (§12 resolves 7/10hr)
             (4, "TWENTY_FOUR_HOUR"),  # 24-hour tariff (e.g. property 709874 — unchanged)
+            (5, "EIGHTEEN_HOUR"),    # off-peak 18-hour — was dropped to "" → STANDARD
             (None, "STANDARD"),      # absent/ND → unknown → standard
         ],
     )